1. Field of the Invention
The present invention relates to a gas sensor including a detection element which is exposed to a gas-to-be-detected in order to detect a particular gas component contained in the gas, and more particularly, to a gas sensor having a protector for protecting the detection element from adhesion of water or the like.
2. Description of the Related Art
A gas sensor has conventionally been known which is attached to an exhaust pipe for use in an automobile or the like. The gas sensor includes a detection element which generates an electromotive force having a magnitude which changes in accordance with the concentration of a particular gas, such as NOx (nitrogen oxides) or oxygen, contained in the exhaust gas, or a detection element whose resistance changes in accordance with the concentration of the particular gas. In such a gas sensor, if a water droplet contained in the exhaust gas adheres to the detection element when heated to a high temperature, the detection element may suffer damage, such as cracking, due to thermal shock. Therefore, a protector covering the detection element is attached to the gas sensor so as to protect the detection element from the adhesion of water (see, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2011-112557
The gas sensor of Patent Document 1 includes a sensor element (detection element); a first protection cover (inner protector) which covers the sensor element and which has a first gas introduction hole permitting a gas-to-be measured (gas-to-be-detected) to flow from the outside of the first protection cover to the inside thereof; a second protection cover (outer protector) which covers the first protection cover and which has a second gas introduction hole permitting the gas-to-be measured to flow from the outside of the second protection cover to the inside thereof; and a gas flow passage which extends from the second gas introduction hole to the first gas introduction hole through a space between the first protection cover and the second protection cover, extends from the first gas introduction hole into the interior of the first protection cover, and reaches a forward end of the sensor element.
Further, in the gas sensor of Patent Document 1, at least one of the first protection cover and the second protection cover has an inner wall member which is solid or has a closed space therein and which is disposed to reduce the width of at least a portion of a flow passage which is a portion of the gas flow passage and which inner wall member extends from the second gas introduction hole to the first gas introduction hole.
3. Problems to be Solved by the Invention
Also, in the gas sensor of Patent Document 1, a through-hole (inner gas discharge hole) is formed in the side wall of a forward end portion of the inner protector so as to discharge the gas-to-be-detected from the inside of the inner protector to the outside thereof. Moreover, a through-hole (outer gas discharge hole) is formed in a bottom portion of a forward end portion of the outer protector which surrounds the forward end portion of the inner protector in order to discharge the gas-to-be-detected from the inside of the outer protector to the outside thereof. Notably, the outer gas discharge hole is formed on the center axis of the outer protector. Also, the forward end portion of the outer protector has a cylindrical side wall and a disk-like bottom portion.
In the gas sensor of Patent Document 1 configured as described above, the gas-to-be-detected taken into the interior of the gas sensor is discharged to the outside thereof through the following route. Specifically, the gas-to-be-detected introduced into the internal space of the inner protector is discharged from the internal space to a side (radially outer side) through the inner gas discharge hole, and flows through the space between the side wall of the forward end portion of the outer protector and the side wall of the forward end portion of the inner protector. Subsequently, the gas-to-be-detected flows through the space toward the axially forward end side, and then flows through the gap between the bottom portion of the outer protector and the bottom portion of the inner protector toward the center axis. The gas-to-be-detected is then discharged to the outside of the protector through the outer gas discharge hole formed in the bottom portion of the outer protector.
However, when the above-mentioned gas discharge passage is employed, difficulty is encountered in introducing a gas-to-be-detected into the internal space of the inner protector efficiently and quickly and in discharging the thus-introduced gas-to-be-detected to the outside efficiently and quickly, whereby the responsiveness of the gas sensor may deteriorate.
Particularly, in recent years, strict regulations have been imposed on exhaust gas emission from internal combustion engines, and accordingly, a demand has arisen for a gas sensor capable of monitoring a difference in exhaust gas pressure between cylinders. Although such an inter-cylinder difference may be monitored by employing gas sensors provided on the respective cylinders, employing a plurality of gas sensors leads to an increase in cost. Therefore, in a known method, a single gas sensor is provided on an exhaust gas assembly of a multi-cylinder engine for monitoring the air/fuel ratio in the respective cylinders. The gas sensor employed in such a method must quickly respond to the exhaust gas originating from each cylinder.